The technique of prospecting by performing a seismic survey in a borehole consists in placing seismic transducers (transmitters and receivers) in a vertical or horizontal borehole, the transducers being capable respectively of transmitting or receiving soundwaves propagating within the geological formations through which the said borehole passes. However, the waves that it is desirable to record are often disturbed by so-called "Stoneley" waves which constitute a particular mode of soundwave propagation due to the presence of the borehole. In general, Stoneley waves propagate inside a borehole or in the proximity thereof, and in a direction that is parallel to the axis of the borehole.
Although numerous digital filtering methods exist to attenuate the influence of these interfering waves, it may be preferable to attenuate the waves in the borehole itself so as to record the desired wave only thus making it possible to use the entire available dynamic range for useful recording.
As described by M. N. Toksoz, C. H. Chen, and M. E. Willis in "Seismic waves in a borehole. A review": Full Waveform Acoustic Logging Consortium Annual Report, 1983, the attenuation factor 1/Q of a Stoneley wave is given by: ##EQU1## where 1/Q.sub.a and 1/Q.sub.b are the attenuation factors of compression waves and of shear waves respectively in the formation surrounding the borehole, and where 1/Q.sub.c is the attenuation factor of compression waves in the fluid in the borehole (water, mud, etc.).
By way of example, FIG. 1 shows how the values of a, b, and c vary as a function of frequency f in a so-called "fast" formation, i.e. where the shear wave propagation velocity V.sub.S is greater than that of compression waves in the fluid. It may be observed that the major coefficient is c. FIG. 2 shows the same parameters for a so-called "slow" formation where V.sub.S is less than the velocity V.sub.f in the fluid. It may be observed that the coefficient c is still the major coefficient at low frequencies.
As a result, if the compression waves are attenuated in the fluid, then the Stoneley waves will also be attenuated not only at any frequency in association with a fast formation, but also at low frequencies even with a slow formation. For a formation that is slow at high frequencies, the attenuation achieved by attenuating the compression waves in the fluid will not be so effective. However, that is of less importance since the Stoneley wave is then less troublesome: first, because it is less excited, and second, because b and 1/Q.sub.b are both high so that attenuation takes place anyway.
Thus, the technical problem to be solved by the present invention is to provide apparatus of the kind specified in the preamble that is capable of attenuating Stoneley waves in a borehole by attenuating compression waves in the borehole fluid.